It is well known that process lines may be purged (i.e., flushed and cleaned) using an air/solvent/air protocol. For example, U.S. Pat. Nos. 4,902,352 and 5,322,571 disclose generally that process lines employed in painting systems may be purged by alternately flowing solvent and air through the lines to remove residual material which is exhausted to a sump. (Please see in this regard, the '352 patent at column 1, lines 10-21 and the '571 patent at column 1, lines 28-34.)
Recently, however, techniques have been proposed whereby additives may be injected directly into a flow of thermoplastic material so as to effect changes in that thermoplastic material's properties (e.g., in terms of color, physical and/or chemical properties, depending on the particular additive employed) which, in turn, effects changes in the properties of the resulting shaped articles, for example, synthetic melt-spun filaments. (Please see in this regard, commonly owned, U.S. Pat. No. 5,800,746, issued Sep. 1, 1998, which claims priority of U.S. Provisional Application Ser. No. 60/012,794 filed on Mar. 4, 1995, the entire content of which is expressly incorporated hereinto by reference.) By use of such techniques, the properties of the filaments may be changed periodically without necessarily stopping the filament production equipment thereby avoiding costly equipment down time to change from one product recipe to another.
When changing from one product recipe to another, however, it becomes necessary to purge the process lines so as to recover to as great an extent as practical the previously supplied additive. In addition, the recovering and purging process of the previously supplied additive is most preferably accomplished simultaneously with the supply and injection of another additive using a stand-by additive supply system. In this way, the polymer extruder and its associated filament spinning line are not required to be shut down during additive change-over.
It would therefore be highly desirable if a system was provided which enabled substantially complete recovery of an additive for a thermoplastic material simultaneously during supply and injection of another additive into a molten stream of the thermoplastic material. It is towards fulfilling such a need that the present invention is directed.
Broadly, according to the present invention, systems and methods are provided whereby additive may be recovered in a primary additive supply system simultaneously with the supply and injection of an additive from a secondary additive supply system. Each of the primary and secondary additive supply systems is provided with respective recirculation lines so as to prevent sedimentation of the additive. According to the present invention, therefore, a protocol is employed whereby substantially all of the additive in the additive supply system, including that quantity of additive in the recirculation lines, may be recovered so that a fresh supply of additive (which oftentimes is different from the previous additive) may be connected for injection into the molten thermoplastic stream at the appropriate time.
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.